1. Field of the Invention
The present invention relates to a method and apparatus for generating a single-sideband optical frequency comb for precisely measuring an unknown optical frequency.
2. Description of the Prior Art
In the area of optical communications as well as in the field of measurement standards, there is a strong need to establish technology for precisely measuring optical frequencies in the 1500 nm band in which there are no optical frequency measurement standards. The increase in the number of multiplex frequencies has been driving the exploitation of new frequency bands and the development of dense wavelength division multiplexing (D-WDM) systems in which the frequency interval is decreased The Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T) recommends 50 GHz (wavelength of approximately 0.4 nm) as the minimum interval for an optical carrier frequency grid. Increasing the frequency bandwidth utilization efficiency by decreasing the frequency interval places more rigorous demands on the stability and precision of each of the carrier frequencies, requiring higher precision and stability with respect to reference lasers and techniques of measuring carrier frequencies
In the prior art, methods of measuring optical frequency include a method that uses an interferometer to measure the ratio between two frequencies, and the beat-down method that measures the difference between two frequencies. The beat-down method, which offers better precision and is widely used for measuring electrical frequencies, combines a known reference frequency with the unknown optical frequency and measures the beat frequency thereof.
If fo is the reference frequency and fx the unknown frequency, combining the two gives a beat signal that is the difference frequency (|foxe2x88x92fx|). If the beat frequency is in the millimeter or lower range, measurement is possible using a frequency counter or the like. However, the bandwidth of the optical detector limits the range of the beat frequency measurement to around 60 GHz.
Research was conducted into a technique of accurately measuring the difference between two laser frequencies that could be used in the terahertz (1012 Hz) range. The method involves placing an optical modulator inside a cavity on the reference optical path to generate optical frequency comb spectra having equidistant comb lines with a spacing that only uses components that are integer multiples of modulation frequency fm. The comb spectra function as a grid on the frequency axis
In measuring difference frequencies using the optical frequency comb, a comb of foxc2x1kfm (where k is an integer) from reference frequency fo is generated and beat frequency xcex94f measured. If it is the k-th comb line that contributes to the generation of the beat frequency, then |foxe2x88x92fx|=xcex94f+kfm, enabling unknown frequency fx to be established from the beat frequency, comb order and modulation frequency.
Research has been carried out into a method of generating wide-band optical frequency combs from a reference frequency laser using an electro-optic (EO) modulator inside a Fabry-Perot cavity. However, the fact that the comb is a double-sideband signal with a carrier is disadvantageous in terms of utilization efficiency, Because there is a sharp falloff in efficiency unless the resonance conditions are satisfied with respect to the reference laser and the modulation and cavity frequencies coincide, in practice there has been constraints on effecting coincidence with the cavity frequency or integer multiples thereof. The frequency span thereof has been limited by the decrease in the optical intensity of high-order combs caused by electro-optic crystal dispersion. Other systems in addition to the above have been proposed that use acousto-optic modulators in an optical fiber loop, but with theses it is difficult to generate combs with a frequency span of more than a few gigahertz.
An object of this invention is therefore to provide a method and apparatus for generating a single-sideband optical frequency comb that is suitable for optical information and communication fields and has an optical fiber structure that is easy to handle.
Another object is to provide a method and apparatus for generating an optical frequency comb that enables optical frequency combs to be generated with a high degree of flexibility.
To attain the above object, the present invention provides a method for generating a single-sideband optical frequency comb, comprising: placing an optical modulator and an optical amplifier in an optical fiber loop, using the optical modulator to generate a sideband signal on only one side of an input signal, and using the optical amplifier to compensate for optical loss while the signal is circulated in the fiber loop to thereby generate at output only a grid of frequency axis components over a wide frequency span.
The object is also attained by an apparatus for generating a single-sideband optical frequency comb, comprising; an optical fiber loop, an optical modulator provided in the optical fiber loop that generates a sideband signal or only one side of an input reference signal, and an optical amplifier that compensates for optical loss by the generated signal that is circulated in the optical fiber loop to thereby generate at output only frequency axis grid components over a wide frequency span.
The optical modulator used may have a dual-electrode construction.
Since the invention uses an optical fiber configuration, it can be readily applied to optical communication systems. Moreover, since there is no cavity control requirement, the modulation frequency can be set as desired, making it possible to generate optical frequency combs with a high degree of flexibility,
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.